Soil health in container gardening

One slightly warmer and sunnier day at a time spring is arriving, which means it is… drumroll please… planting season! This means it is time to get all hands working and all plants in the dirt.

For me, getting the first soil out feels like such a breath of fresh air. The smell, the texture, the heaviness of a big bag, everything about it feels glorious. On a piece of land, getting soil to optimum health can be a very tricky and lengthy process. It can take years of manipulation to get good results. With container gardening it is much easier because you can handpick from the beginning what you are working with, and the smaller amount of soil makes manipulation very doable. However, it is still a process that can take patience and experimentation to get great results.

Step 1. Planning out what you will need

Documenting your journey will make the process of creating, using, and reusing optimal soil much smoother and more effective. The route to soil health may be a lot of trial and error and adjusting with each season, so recording exactly what you’re doing will be invaluable. Especially when 6 months down the line you need to remember what you used and you totally forgot. Before picking or making your soil you will need to know how much you need and for what you need it. Observe the pots you will be growing in and estimate how much soil will be needed to fill them. Once you have planned out what you are going to grow, do a little research on the soil conditions that each plant prefers and document this. Tomatoes, for example, need a lot of nitrogen so choosing a soil high in nitrogen or building your own soil with lots of nitrogen fixing bacteria (obtained from compost) would be best. Want to know more about these bacteria and the basics of soil? Check out my previous article here. Another aspect to look at during this research is acidity. Potting soils can be found in a range of acidities and different plants prefer different ranges. On the more acidic end of the spectrum are blueberries which prefer a ph of 4.5-5.5, and on the more basic end are beets and asparagus, which do well in the range of 6-8. Most other vegetables, herbs, and fruit prefer slightly acidic soil (around 6) and will be fine in your average soil, but it is worth noting and attempting to adjust if they are not growing well.

Step 2. Acquiring the right soil

Buying soil can be an overwhelming process as there tends to be a lot of options. Potting soil is the best for container gardening as it offers a lot of nutrients and holds water well in a small space. Starting out with a basic one and adjusting based on the needs of your plants and how they grow is the simplest way to go. Read the ingredients on the bag and look for percentages of ingredients (for the tomatoes look for a high nitrogen content). Also look for ones that contain the fungi for mycorrhizal associations. Alternatively, you can mix your own with a very simple recipe.

For seeds: when starting from seed, you don’t want to overwhelm them with a lot of minerals. Mix two parts compost (store-bought or homemade) with two parts coconut coir and one part perlite. The coconut coir is great at retaining water and is a sustainable alternative to peat moss. Perlite is the small white particles we associate with potting mix, and although it is non-renewable, its mining has little effect on the environment and less than 1% of the world’s supply has been mined. As an alternative, composted sawdust or chipped bark will have the same effect. Mix the ingredients well to get an even distribution and store in a lidded container in a cool, dry place. When planting, be sure to first thoroughly wet the mixture. This can take some patience if the mixture is very dry as water will initially roll right off the top.

For plants: Combine two parts compost with one part coconut coir and a little perlite for drainage (about two handfuls per 45 liters). Another two parts or less of worm castings can be added for plants that require a lot of nutrients as well as a mycorrhizae starter. If planting something that will stay in the same container for a long period of time, like a bush or perennial vegetable, buy a loam soil and combine that with an equal amount of compost.

Step 3: Adjusting your soil

Now that you have your basic soil ready, it is time to consider adding in some extra ingredients. You may notice earthworms getting into your potted plants depending how close you are to the ground – if you do that is a good indicator of healthy soil. However, for most container gardening scenarios adding worms in is not necessary. If you have raised beds or very large containers, a few earthworms can be a beneficial addition. Microorganisms should be in the soil if you have used a good quality compost. To maintain their health, keep the soil well-watered and avoid using chemicals of any kind. Microbes thrive in moderate temperatures and moist conditions and do not tolerate chemicals. As for mycorrhizae, if making your own soil, the compost should already contain fungi for mycorrhizal associations. But if you want to make sure you have it, you can buy a container of fungi spores. Glomus inraradices is a fungus that most herbaceous plants are very happy with – just read the directions on the container about how much to add. Mix it in when combining everything else, and the plants and fungi will do the rest! If your plants do not grow well the first season, do a little more research on what they prefer soil-wise and adjust accordingly the next season. This may include adding phosphate, greensand for potassium, blood meal for nitrogen, etc., to supplement your soil. These can be mixed in at the beginning of the next season when you reuse the soil.

Step 4: Reusing your soil

The initial investment can be a lot if you are buying premade potting soil or the ingredients to make your own, but it is not a shopping trip you will have to repeat every year. Reusing soil is easy and sustainable. There are really two points to be aware of: not carrying over disease and replenishing nutrients and minerals. If a plant ended up with a disease, do not reuse the soil from that container, as it too will contain the disease. The same goes for insects, if there was a bad infestation there is a possibility the insects will make it through to the next season. Secondly, some nutrients and minerals will have to be replenished because all of the good food that was in the soil is now in the plant (and in you if you have eaten the plant!) Here’s the basic process. At the end of the season, simply dump your soil onto a tarp or into a large container. Inspect a bit for insects which you will want to pull out along with weeds and any large roots which would take up too much space the next season (small roots, leaves, and other debris are fine and even beneficial for the added organic matter). Store it in a closed container in a cool dry place until the next season. When it is time to grow again, mix one part fresh compost with one part fresh potting mix and two parts of the reused soil along with any additions you think are necessary.

Note 1: this ratio is not set in stone, and the amount of fresh ingredients you add in will depend on the quality of the initial soil along with how hungry of a plant lived in that soil. This is one area where writing down the process and observations will be very useful.

Note 2: It is advised to never use the same soil for tomato plants in consecutive seasons. This is because tomatoes are particularly prone to disease and even if they weren’t diseased the first season, there could be something there that will affect them the next one. So keep tomato soil separate from the rest and reuse it for other plants, after which it will be fine to again use with tomatoes!

Photo by Carlin Roland

Simplicity for a peaceful mind and garden

As mentioned before, there are many options for soil and new studies continually bring different recommendations for optimal soil health.

One of the beautiful things about gardening is that it brings simplicity back into our lives, and therefore not overthinking or overdoing is important.

Following a few basic principles and making slight adjustments as you go will put the overdoing at bay and is the best way to learn. It is always best to go the natural route in order to make sure the microorganisms are happy, so carefully read potting mixes, fertilizers, etc. to make sure they are as chemical-free as possible is important.

Watering is one of the most obvious part of gardening, but new-found appreciation can be had when imagining the balance of air and water among the minerals and thinking about what materials will aid in that balance. Once you feel comfortable with the basic soil setup, experimenting with perlite vs. sawdust vs. bark chips for air flow and different amounts of coconut coir or alternatives for water retention are ways to customize your soil more precisely for different plants. Because just as soil is not static, neither are plants, and they all have their different needs. Start with the basics – good compost, good soil texture, a balanced amount of water. Then sit back and watch, and record, planning for what may need to be changed the next time around. And if you’re sitting back, watching and nothing ever grows, it is A-okay. This just means that you have more time to think and plan for the changing part. Because at the end of the day, gardening is really just a dirty, wonderful, eventually life-producing experiment.

Growing up in New Hampshire in the US, nature was intertwined in Carlin’s life from the start. While studying Biology, she worked at an organic vegetable farm in the summers. In 2017, she completed a permaculture course and internship in Costa Rica. She is currently in London working on an Msc in Plant and Fungal Taxonomy, Diversity, and Conservation.